Many industries, such as the pharmaceutical and medical packaging industries, require aseptic packaging of items that must remain sterile for as long as they are packaged. Therefore, the ability of the package material to act as an adequate bacterial (and other infectious organism) barrier, and the ability of the package seal to remain intact and continuous are important considerations.
The sterility of a package is typically evaluated by the medical professional prior to use by visual inspection of the package surfaces and the peeled sealing surfaces after the package is opened. A puncture in the package or a non-uniform peeled sealing surface indicates that the package contents may not be sterile and should be discarded. A uniform frosty appearance of the peeled sealing surfaces generally indicates a good seal.
The frosty appearance is typically the result of the adhesive, which is used to bond the packaging materials together, being left on both of the sealing surfaces after peeling. When the adhesive is left on both of the sealing surfaces in this manner, a cohesive separation has occurred within the adhesive layer during peeling. Alternately, an adhesive failure may occur between the adhesive and one of the sealing surfaces. When there is an adhesive failure, one of the peeled sealing surfaces has the frosty appearance of the adhesive, and the other sealing surface has the smooth appearance of the bare packaging material.
Thus, when the frosty appearance of a cohesive separation is observed, the person opening the package is assured that the adhesive seal was continuous and adequately bonded to both of the sealing surfaces. This provides confidence that the item within the package is still sterile. However, when the smooth appearance of an adhesive failure is observed, the person opening the package does not know whether the seal was good and that an adhesive failure occurred during peeling, or that the seal was bad and that an adhesive failure occurred prior to peeling the package open. If an adhesive failure occurred prior to peeling, then the package contents may not be sterile. Because it is typically not a simple matter to conclusively determine whether the adhesive failure occurred before or during peeling, it is most prudent to assume that the adhesive failure occurred prior to peeling, and discard the contents. This, of course, increases the cost associated with the use of the sterile items.
One type of adhesive peel defect is typically called "branching," or "channeling." The defect is called branching because there are smooth adhesive failure areas surrounded by frosty cohesive separation areas within the peeled areas. The smooth adhesive failure areas look like the branches of a tree. If the branches of the adhesive failure areas connect from the outside of the seal area to the inside of the seal area, then a channel has been formed through the seal. A single path of adhesive failure may be termed a channel defect. As used herein, "branching" refers to both branching and channeling type defects.
Branching defects have been observed on packaging made from a polyester of terephthalic acid, ethylene glycol, and 1,4-cyclohexanedimethanol (PETG), that has been coated on at least one side with silicone. Branching peel defects are not typically found on packages made from PETG that has not been coated with silicone. The silicone coating is useful as an anti-nesting agent, or in other words to keep stacked pieces of the PETG from sticking to each other. Therefore, it is desirable to continue using the silicone coating.
Contamination on the surface of the packaging material tends to create a defect that looks like branching, but which is actually quite different. Surface contamination inhibits a proper seal from ever forming between the adhesive and the surface of the package material. Thus, with surface contamination, a proper seal is typically never formed. By distinction, a branch defect looks like there may have been a gap between the adhesive and the packaging material, but in reality there was a good adhesive seal.
It has been thought that adhesive peel defects such as branching could be eliminated by increasing the adhesion between the PETG packaging and the adhesive. An electrostatic or corona discharge applied over a polymeric surface tends to oxidize the polymer surface, increasing the surface energy and polarity, and promoting adhesion to the polymer surface. Therefore, attempts have been made to treat packaging materials for sterile items with an electrostatic discharge, in an effort to eliminate branching peel defects. However, such treatment has typically not reduced branching peel defects effectively.
What is needed, therefore, is a method for achieving reduced branching peel defects in adhesive bonded plastics.